Document
2019-01-1921
Ice-Crystal Icing Accretion Studies at the NASA Propulsion Systems Laboratory
Peter Struk, Juan Agui, Michael King, and Thomas Ratvasky
NASA John Glenn Research Center
Tadas Bartkus and Jen-Ching Tsao
Ohio Aerospace Institute leading to icing but in an external flow environment. The desired
Abstract
conditions to be controlled include (1) wet-bulb temperature, (2) particle size distribution, and (3) melted portion of incoming ice.
This paper describes an ice-crystal icing experiment conducted at the However, it has been difficult to vary these parameters independently.
NASA Propulsion System Laboratory during June 2018. This test As will be discussed, the relative humidity in the plenum was the main produced ice shape data on an airfoil for different test conditions parameter varied which affected multiple icing parameters in the test similar to those inside the compressor region of a turbo-fan jet engine.
section. The key icing parameters being measured at the test section Mixed-phase icing conditions were generated by partially freezing out include the total water content, melt ratio, particle size distribution, and a water spray using the relative humidity of flow as the primary aero-thermal conditions such as pressure, velocity, temperature, and parameter to control freeze-out. The paper presents the ice shape data humidity.
and associated conditions which include pressure, velocity, temperature, humidity, total water content, melt ratio, and particle size This paper describes an experiment on ice-crystal icing using PSL that distribution. The test featured a new instrument traversing system took place in June 2018. The primary objective of this test was to which allowed surveys of the flow and cloud. The purpose of this work generate a set of ice shapes on an airfoil model under well- was to provide experimental ice shape data and associated conditions characterized conditions to help develop and validate ice-crystal to help develop and validate ice-crystal icing accretion models. The accretion models. A NACA 0012 airfoil was used for this testing as it results support previous experimental observations of a minimum is well characterized for ice accretion from extensive testing in melt-ratio threshold for accretion to occur as well as the existence of a supercooled liquid water (e.g. [11, 12]) thereby eliminating the need to plateau region where the icing severity is high for a range of melt characterize certain aspects of the airfoil. The ice shapes were ratios. However, a maximum limit for melt ratio, which is suggested generated across a series of four different flow conditions where the in the ice crystal icing literature, was not observed perhaps relative humidity in the plenum was the primary parameter varied. In complicated by the potential for some supercooling of the water at addition, tests were conducted at an angle of attack (AOA) of 0º these conditions.
and -4º.This paper presents four ice accretions at different melt ratios for one flow condition at 0º AOA. The accretions at other flow
Introduction
conditions and AOAs are planned to be reported at a later date.
However, some results from those conditions can be found in Some jet-engine power-loss events are attributed to icing that occurs complementary papers presented in this conference which provide due to the ingestion of ice-crystals at high altitudes [1]. NASA is more details of instrumentation [13] or use the data for model investigating the fundamental physical mechanisms of such icing comparison [14].
which occurs in core compressor regions of jet engines [2]. The primary goal of these investigations is to improve understanding of the The June 2018 testing built upon results from a similar testing at the ice growth physics and expand engine aero-thermodynamic modeling PSL in 2016 [15]. Analysis of the 2016 test results showed that there capability to predictively assess the onset and growth of ice in current were radial variations in flow and cloud properties due to the spray bar and future engines during flight [3-6].
patterns selected. Since not all measurements were made at centerline in 2016, some key flow and cloud properties needed to be extrapolated The NASA Propulsions Systems Laboratory (PSL) has recently added from the available data to estimate conditions at the ice accretion site.
a capability to inject ice particles into an operating jet-engine [7]. To For the 2018 PSL testing, an instrument traversing system was date, a number of successful test programs using both jet engines and developed which allowed direct measurements of centerline conditions test rigs have been performed at the facility offering insight into the at the accretion sites as well as allowing the ability to perform flow engine ice-crystal icing [8-10]. At PSL, the ice particles are generated surveys at various positions within the cloud. Finally, there were using liquid-water spray nozzles which are injected upstream of the multiple additional objectives of the June 2018 PSL which are not test section in a plenum area. The water droplets freeze prior to described in this paper.
reaching the test section due to a combination of convective and evaporative cooling. Once inside the jet-engine, the ice particles are
Experimental Description
presumed to break-apart and begin to melt generating a mixed-phase condition before reaching the accretion site in the compressor.
This section highlights changes in the 2018 experimental setup from those in 2016. The 2018 experiment includes a new instrument Due to spatial and access constraints with respect to instrumentation, traversing system.
it is difficult to study the physics of such accretions directly inside the engine. Thus, NASA is simulating the internal engine conditions Page 1 of 11 SEA Multi- Element [also known as the Multi-Wire (MW)] Probe and Airfoil Model Ice Crystal Detector (ICD), Rearward Facing Probe (RFP), and a Rosemount Total Air Temperature (TAT) probe. Results from the ICD The NACA 0012 airfoil model has a 266.7 mm (10.5 inch) chord are not presented in this paper.
length and has been previous described [15]. The 2016 testing included surface polyimide heaters on the solid aluminum extension to prevent ice from forming on the extensions. For the 2018 tests, the heaters were SEA MW removed due to the tendency for the heaters to cause undesired Rosemount TAT asymmetric ice shedding. Furthermore, the aluminum extensions were RFP black anodized for the 2018 tests to allow for good imaging contrast SEA ICD for ice that forms on the extensions.
Traversing System An instrument and test article traversing system was developed for this test. The system included four separate traversing mechanisms specifically for: (1) intrusive probes, (2) non-intrusive probes, (3) airfoil / test article, and an (4) isokinetic, or canister-type, probes. The overall installation integrated inside the PSL facility is shown in Figure 3. Solid model image showing intrusive probe traversing Figures 1 and 2.
mechanism. The probes traversed in this test are listed in the figure.
The non-intrusive probe mechanism (Figure 4) included the Artium High Speed Imaging (HSI) and Phase Doppler Interferometer (PDI) instruments. These instruments measured the particle size distribution in a plane approximately 66 cm (26 inches) aft of the duct exit plane.
The measurement sample volumes (approximately a small point in space) for the PDI was 1.9 cm (0.75 inches) left of the vertical duct centerline looking upstream while the HSI was 1.3 cm (0.5 inches) right of the centerline. The sample volume of both probes could traverse vertically from the horizontal duct centerline to about 22.9 cm (9 inches) above the centerline as seen in the figure.
Traversing Region Figure 1. Solid-model image of the traversing system installation inside the PDI Transmitter PDI Receiver PSL.
HSI Receiver HSI Transmitter Non-Intrusive Intrusive Sample Volumes Probe Probe Traverse Traverse Duct Exit Figure 4. Solid model image showing the intrusive probe traversing mechanism. For the current test, the non-intrusive probes included the Artium HSI and PDI.
The NACA 0012 airfoil test article, which is described in more detail elsewhere [15], was moved into and out of the flow using the Airfoil / Test airfoil/test article traversing mechanism (Figure 5). The leading edge Article Traverse of the airfoil was approximately 12.7 cm (5 inches) downstream of the duct exit plane. The airfoil was nominally stowed outside of flow Figure 2. Solid-model image showing a closer view of the traverses for below the tunnel duct (as depicted in Figure 2) and was traversed into the intrusive probes, non-intrusive probes, and airfoil / test article.
the flow centerline for ice accretion tests. The titanium airfoil center body was allowed to thermally stabilize before the ice cloud was Solid-model images of each separate traversing mechanism are show activated by monitoring the thermocouples on the airfoil. After the ice in Figures 3 through 6. The intrusive probe mechanism (Figure 3) accretion test, the airfoil was moved out of the flow to allow for deicing allowed a survey of the upper half of the test section with the sample and cloud diagnostics.
volumes of the probes positioned approximately 12.7 cm (5 inches) downstream of the duct exit plane. The intrusive probes included the Page 2 of 11 The Isokinetic Probe (IKP2) [16, 17] was traversed into and out of the measurement results presented are averages for the duration of the flow using the traversing mechanism shown in Figure 6A. The probe spray. Measurement of temperature and humidity at the test section was stowed above the duct at the horizontal centerline and could be include both the cloud-off and cloud-on values. In some cases, the traversed to any location of the duct. The probe inlet was 31.8 cm cloud-on data is reported as a change (i.e. delta, ) from the cloud-off (12.5 inches) aft of the duct exit plane. The probe had two background measurement. The cloud-off measurement was the 30-second average humidity (BH) inlets, one on the canister as shown in Figure 6B value just prior to cloud activation. The cloud-on values were 30- (BH-1), and the other on the top of the inlet as seen in Figure 6C second averages of the measurements just prior to cloud deactivation.
(BH-2). The horizontal and vertical locations of the BH-1 relative to Drift of the plenum conditions was accounted for by adjusting the the probe inlet was 8.9 cm (3.5 inches) left and 1.9 cm (0.75 inches) cloud-off values by the same amount that the plenum values drifted above, respectively. For BH-2, the vertical position was 5.7 cm (2.25 during the spraying event.
inches) above the probe inlet but at the same horizontal location. When post-processing the data, both background humidity inlets were Temperature and humidity measurements were made at the duct exit considered and the average value between the two used in the plane using a Rearward Facing Probe (RFP) [18]. In addition, a calculation of total water content ( TWC) . If a background humidity humidity measurement was made in the plenum - the inlet for this measurements became contaminated, as would occasionally occur, measurement was located on the upper wall of the plenum upstream of then that measurement was not used.
the spray bars. The temperature measurement was made using a resistive temperature device (RTD) embedded in the RFP. The humidity was measured using a SpectraSensor instrument (Model Titanium Airfoil WVSS-II) which uses an optical absorbance technique on the air drawn Traversing Center Body into the RFP. An accompanying paper [13] provides further details of Range the RFP used in the current tests.
Cloud Measurements Aluminum The cloud measurements included TWC , liquid water content ( LWC ), Airfoil Extensions and particle size distribution (PSD). The TWC was measured using the IKP2 while the LWC was measured using the MW probe. The PSD was measured using the Artium HSI and PDI probes. Details of the HSI and PDI techniques used to extract the PSD are described elsewhere [19].
Test Conditions and Procedure Figure 5. Solid model image showing the airfoil / test article traversing mechanism. For the current test, the airfoil test article was a NACA For these tests, the mixed-phase conditions were generated by partially 0012 airfoil. The airfoil is shown in the retracted position below the freezing the liquid-water droplets ejected from the spray bars. The duct.
primary parameter used to control the amount of freezing was the relative humidity in the plenum ( RH ). Note that total conditions exist 0,i A B in the plenum due to the slower moving flow. Therefore, the relative humidity is evaluated using total conditions (subscript “0”) in the plenum (subscript “i” for inlet plane). In these tests, RH was varied 0,i from approximately 0% to just over 50%. The humidity variations, or sweeps, occurred at four different test conditions as outlined in Table 1, although only test condition II results are reported in this paper. The Background total temperature ( T 0 ) was maintained constant for each test condition Humidity Inlet 1 at 7.2 ºC (45 ºF) in the plenum. However, the flow and cloud thermally Background C interact which required a total temperature measurement with the Humidity Inlet 2 cloud-on, 𝑇 , using the RFP. Table 1 also shows the cloud-off static , temperature ( T s ) and static pressure ( P s ) for the four primary conditions 5.7 cm of 2018 PSL test. It is a combination T , P , and humidity (i.e. the wet- s s bulb temperature) that affect the different melt ratio and TWC clouds achieved and required characterization for each test condition. The Probe Inlet individual droplets tend towards the static wet-bulb temperature (T ) wb,s as they travel from the spray bars to the test section. T wb,s is evaluated Figure 6. Solid model image showing the traversing mechanism for the using static conditions.
isokinetic probe (A) and background humidity inlets on the probe (B and C). Both the probe and traversing mechanism were developed by The cloud was generated in the same manner as during the 2016 test Science Engineering Associates.
[15] and only the key aspects are repeated here. A total of 22 and 19 nozzles were selected from the Standard and Mod-1 set, respectively, Aerothermal & Humidity Measurements and the spray bar patterns remained the same for each nozzle set. The TWC e,T target (denoted by the subscript “T”) at the duct exit plane Measurements of air temperature, pressure, and humidity were made (denoted by the subscript “e”) was set to 2 g/m . The value was set by at various locations in the plenum and at the test section. All test adjusting the injected water mass flow rate assuming no evaporation section values reported are centerline values. The plenum and uniform mixing across the center 61 cm (24 inch) diameter of the Page 3 of 11 PSL exit duct. The PSD at the PSL spray bars (i.e. inlet plane) is particle sizes were larger than IRT presumably due to the smallest approximated to be those measured at the NASA Icing Research particles evaporating away although the final distributions did not Tunnel (IRT) for the same supplied water and air pressures [15]. In this signicantly vary with the different plenum relative humidity values.
paper, a particular spray PSD is referred to by the IRT median volumetric diameter and is given the subscript “i” to denote the inlet or spray bar plane (e.g. MVD i ). Since evaporation does occur for these tests and larger particles tended to concentrate near the center of the duct, the actual TWC and PSD at the test article were measured and are given the subscript “m”.
Table 1. Target cloud –off test conditions. Only test condition II results are reported in this paper.
Test U P P T T e 0 s 0 s Condition # (m/s) (kPa/psia) (kPa/psia) (ºC) (ºC) I 85 44.8 / 6.5 42.8 / 6.21 7.2 3.6 II 135 44.8 / 6.5 39.9 / 5.79 7.2 -1.8 Figure 7. Humidity and air temperature changes at the test section III 185 44.8 / 6.5 36.0 / 5.22 7.2 -9.8 centerline as a function plenum relative humidity.
IV 135 87.6 / 12.7 78.05 / 11.32 7.2 -1.8 Table 2. Select particle distribution measurement values for test condition II at Ice Accretion Image Analysis Method 5 different plenum humidity values. For comparison, the IRT values at the same spray bar settings are included in the table.
The video image analysis method has been described in detail previously [15, 20, 21]. The same image analysis methods were used RH d d d d d d PL v0.10 v0.25 v0.50 v0.75 v0.90 v0.99 to document ice accretions in the present study.
% μm μm μm μm μm μm
Results
9 12 20 33 48 99 IRT 0.6 18 25 36 52 71 104 The results are divided into test condition and ice accretion sections.
The test condition section is further divided into aerothermal and cloud 19.6 19 26 36 53 74 112 subsections. For this paper, only the results from test condition II are 32.8 18 25 35 49 68 100 presented.
49.6 17 24 33 48 67 99 Test Section Conditions For water content, the data presented are averages of the instantaneous water content measurements during stable portions of the spray.
The conditions at the duct exit plane of PSL (denoted with the subscript Averages are used since the TWC and LWC signals tend to fluxtuate “e”) represent the conditions at the accretion site.
about a mean value under steady state conditions. Figure 8 shows the TWC and LWC averages as a function of RH for test condition II. The 0,i Aerothermal Conditions figure shows measurements from the IKP2 and the three heated elements (2.1-mm, 0.5-mm, and a half-pipe, HP, which is a 2.1-mm The changes in humidity and total air temperature between cloud-on scoop) of the MW probe [22]. All data of the MW is presented and cloud-off conditions at the test section centerline as a function of although only the results of the 2.1-mm wire and IKP2 results are used RH 0,i (plenum humidity) are presented in Figure 7. This figure presents in the analysis, as explained later. The IKP2 and MW measurements humidity data as changes in mass-mixing ratio at the test section were taken at different times and with different resolution in RH . The 0,i ( MMR ). The MMR is the ratio of the vapor to dry-air mass. The e IKP2 data was generally linear with RH 0,i and a curve fit, shown in humidity data in Figure 7 come from the RFP and IKP2 BH. This Figure 8, was used to interpolate TWC values in between measured figure shows that increasing RH 0,i suppresses evaporation resulting in points. Recall that the injected water mass flow was set to produce a smaller humidity changes at the test section when the cloud comes on. 3 2 g/m cloud if distributed uniformly across the center 61-cm (24-inch) Correspondingly, there is less evaporative cooling with increasing diameter area without evaporation. The resulting measurements show RH 0,i resulting in smaller changes in total temperature at the exit ( T e ), water contents above 2 g/m even under conditions where significant as can also be seen shown on the right axis of Figure 7. An evaporation is occurring (i.e. RH 0,i = 0%). These elevated TWC accompanying paper [13] provides further details on the reduction of measurements suggest that the water is concentrating more at the data from the RFP.
centerline.
The ratio of the LWC to TWC is the melt ratio, MR , and details of this Cloud Conditions calculation have been previously describe [15]. The equations, (1) – (3), are repeated below for clarity. The LWC is calculated from This section presents the measured PSD, LWC , and TWC . The PSD the 2.1-mm element, where the subscript denotes the particular wire measurements for test condition II at four different relative humidity ( LWC m,2.1 ) of the MW probe while the TWC comes directly from the values are shown in Table 2. The table shows the various percentile IKP2 ( TWC ). For equation (3) related to the MW, the collision m,IKP2 cumulative volume diameters ( d v ) and includes the IRT values for the efficiency ( 𝐶𝐸 ) is the percentage of the upstream particle mass same spray bar settings used in the test. The spray bar settings colliding with the particular hot-wire, the water-catch efficiency ( 𝑊𝐸 ) corresponded to a 20μm MVD cloud at IRT which is approximated to is the percentage of collected water that is evaporated by the hot wire, exist in the plenum prior to any evaporation. At PSL, the measured Page 4 of 11 and the false response ( 𝐹𝑅 ) is the signal on the liquid water elements on the interpolated MMR from Figure 7. Tables 4 through 6 show that due to ice crystals.
it was not possible in the present experimental setup to vary MR,e independently of test section TWC , temperature, and humidity since these also varied with RH 0,i .
𝐿𝑊𝐶 𝐿𝑊𝐶 (1) 𝜂 = = ெோ 𝑇𝑊𝐶 𝐿𝑊𝐶 + 𝐼𝑊𝐶 Table 3. Ice accretion test matrix summary (measured values) Test U P T Case RH Escort 𝑇𝑊 𝐶 = 𝑇𝑊𝐶 = 𝐿𝑊𝐶 + 𝐼𝑊𝐶 (2) e 0 0 PL , ூ ଶ Cond kPa File # m/s (psia) °C % # 𝐿𝑊 𝐶 = 𝐶 𝐸 ( 𝑊 𝐸 𝐿𝑊𝐶 + 𝐹 𝑅 𝐼𝑊𝐶 ) (3) , ଶ . ଵ ଶ . ଵ ଶ . ଵ ଶ . ଵ a 20.6 170 44.8 See b 25.2 593 II 133 (6.5) Table 5 c 30.2 171 For equation (3), the 𝐶𝐸 was calculated from literature data [23, 24] ଶ . ଵ d 34.5 169 to be ~97.6% using the test flow conditions and the d v50 data from Table 2. The PSD variations in Table 2 had less than 1% effect on the Table 4. Test cloud conditions calculation of 𝐶𝐸 . The value for 𝑊𝐸 was calculated to be 59.1% ଶ . ଵ ଶ . ଵ from the data in Figure 8. The value for 𝐹𝑅 was calculated to be RH TWC 0,i e MR,e ଶ . ଵ Case (%) (g/m ) (dimensionless) 6.2% using data (not presented) at the same flow and PSD but significantly colder air temperatures where the cloud was expected to Source Measurement Fig. 8 Fig.8 be fully glaciated. Using equations (1) – (3), the calculated values of a 20.6 3.3 0.14 MR are plotted in Figure 8. The results show that the cloud transitioned b 25.2 3.4 0.21 from glaciated to liquid conditions as RH 0,i increased.
c 30.2 3.7 0.28 d 34.5 3.8 0.51 Table 5. Test temperatures conditions Case T 0,i (=TPL) T T T T T 0 0,e s,e wb,0,e wb,s,e ( C) ( C) ( C) ( C) ( C) ( C) Fig.
Meas.
Source Calc. Calc. Calc. Calc.
WE =59.1% 2.1 a 7.1 -6.6 0.5 -8.3 -2.6 -6.8 b 7.3 -6.1 1.2 -7.6 -1.9 -6.0 c 7.6 -5.5 2.1 -6.8 -1.1 -5.1 d 7.2 -5.1 2.2 -6.6 -0.8 -4.7 Figure 8. Measured water contents as a function of plenum relative Table 6. Test humidity conditions humidity for test condition II. The figure includes the calculated water catch efficiency, WE, for the 2.1-mm wire at this test condition.
Case 𝑀𝑀 𝑅 MMR MMR e (g/kg) (g/kg) (g/kg) Source Meas. Fig. 7 Calc.
Ice Accretion Characteristics
a 2.9 3.0 5.9 This paper presents ice accretion measurements for test condition II b 3.6 2.7 6.3 at a zero-degree angle-of-attack. The accretions were generated at c 4.4 2.4 6.8 different RH along the transition from mostly glaciated to more 0,i liquid cloud conditions. Attempts were made to study accretions at d 4.9 2.2 7.1 low, mid, and high MR . The specific RH 0,i where ice accretions were studied is shown in Table 3. The table also lists the data acquisition Table 5 includes calculated wet-bulb temperatures both at total ( T wb,0,e ) number from the PSL facility (i.e. the Escort file number) and static conditions ( T wb,s,e ). These calculations use the humidity corresponding to each test. The cases will be referred to by the letter values MMR from Table 6. An interesting observation is that the e as defined in Table 3.
calculated T wb,0,e is cooler than the total temperature while T wb,s,e is The test conditions for each accretion case are shown in Tables 4 warmer than static temperature. The explanation for this is that the through 6. The second row in each table indicates the source of the water vapor saturation temperature corresponding to the measured data. Table 4 shows the cloud conditions for the accretion cases humidity is between the total and static temperatures. When the including the TWC and . These values were calculated by e MR,e humidity is below saturation, evaporation occurs using energy interpolating the data from Figure 8 using the RH 0,i for each accretion resulting in a calculated wet bulb that is below the dry bulb case. The temperature and humidity conditions for each accretion case temperature. However, when the humidity exceeds saturation, are shown in Table 5 and 6, respectively. The temperature at the condensation occurs resulting in heat being deposited and a calculated accretion site was calculated by adding the interpolated T from wet-bulb that is warmer than the dry bulb temperature.
Figure 7 to the measured average total temperature in the plenum, T 0,i (or TPL) during the accretion. The humidity values were calculated Figures 10 through 13 show results from each accretion case studied similarly, namely that the plenum humidity, MMR , was adjusted based i in this paper. Each figure contains images of ice accretions and Page 5 of 11 analysis results. The images are of the ice shapes at select times during 𝑇 value of -2.8 C calculated in Table 5. It would be expected that ௪ , , the accretion process (typically 60, 180, 300, and 600 seconds after the the leading edge temperature of the airfoil approach the total wet-bulb start of the spray). The images are from a span view camera (left temperature ( 𝑇 ) with a small presence of liquid due to ௪ , , columns, images A-D) and side-profile view camera (left center evaporative cooling. However, the temperature of liquid water and ice columns, images E-H). In each figure, there is an Ice Shape Analysis mixture (presumably near 0 C) as well as heat transfer into the airfoil Graph (upper right, image I) extracted from the side-profile. The may explain why the total wet-bulb temperature was not achieved at shadow region denoted in image I represents the minimum ice the leading edge. Interestingly, TC06 did measure exactly the value of thickness which can be measured. This minimum is due to a the total wet bulb temperature of -2.8 C suggesting that some perspective blind spot of the imaging system. Figures 10 through 13 evaporation is occurring from the surface at this location.
also show a Surface Temperature and Ice Growth Rate Graph (lower right graph, image J). There are five surface temperature Figure 11 shows the ice accretion for Case (b), which had a MR,e of measurements presented (left axis, image J) with the first three 0.21 at this condition. In this case, the ice thickness initially grew to thermocouple locations depicted in image I. The remaining approximately 1.7 mm within the first 10 seconds and remained at this thermocouples, TC13 and TC14, are located at X = 80.39 mm and thickness until 150 seconds (Figure 11 J – gray line labelled “t”).
Y = 16.00 mm and at X = 139.07 mm and Y = 13.74 mm, respectively.
Subsequently, the ice began to grow in a linear fashion reaching 7.6 See Figure 4 in reference [15] for a full definition of thermocouple mm at 615 seconds (growth rate ~ 0.013 mm/s per Table 7) when the locations. The ice growth (rightmost axis in image J) is measured at test was ended. The ice shape was pointed and devoid of feather the centerline of the titanium airfoil using the span view camera data.
features which is indicative of significant erosion. A shadow of the ice Linear curve fits of the ice thickness ( t ) versus time, or ice growth rates, particle debris field is visible in the side-profile view camera images during select intervals are provided in Table 7. The purpose of (Figures 11 G & H). Figure 11 J shows that the leading edge presenting this information is to provide data against which to compare thermocouple (TC03) reached 0 C just 7 seconds after the spray models of ice crystal icing.
initiation and remained at this temperature for 120 seconds.
Subsequently, TC03 began to decrease reaching -1.1 C at the end of Table 7. Measured ice growth rates during select time intervals for the test. In this case, the 𝑇 is calculated to be -1.9 C so again the ௪ , , Cases (a) – (d).
leading edge temperatures are slightly warmer than the calculated total Interval wet bulb temperature.
Case / Escort Time Growth Duration Number Interval (s) Rate (mm/s) (s) Figure 12 shows the ice accretion for Case (c), which had an MR,e of 0-100 101 0.000 (a) 170 0.28 at this condition. Compared to Case (b), the ice shape was more 120-499 380 0.004 substantial in terms of size reaching just over 30 mm thickness at the 0-10 11 0.194 leading edge at the end of the test (Figure 12 J – see gray line (b) 593 11-150 140 0.000 labelled “t”). Furthermore, the final ice shape has more thickness 151-615 465 0.013 0-12 13 0.000 perpendicular to the chord with rough, feather-like features 13-35 23 0.086 downstream of a smoother and pointed leading edge. It is (c) 171 36-239 204 0.013 recommended that modelling comparisons be limited to the smoother 240-609 370 0.069 leading edge regions as the downstream feather-like features seen in 0-19 20 0.000 Figure 12 G & H can be anywhere along the span of the airfoil and 20-35 16 0.091 (d) 169 represent a maximum-thickness combined cross section. Occasionally, 36-307 272 0.015 asymmetrical ice shapes and/or feathers appeared due to ice forming 308-636 329 0.081 on the airfoil extensions which then shifted to one side of the airfoil due to aerodynamic forces – one such example can be seen in Figure 12 H where an asymmetric feather appears below the airfoil. The Figure 10 shows the ice accretion for Case (a), which had an MR,e of thermocouple data for Case (c) (Figure 12 J) shows that the leading 0.14 and was the lowest melt ratio accretion test at this condition. In edge reached 0 C quickly (within 6 seconds) and remained at this this case, there was only a small accretion (~ 2 mm thickness) on the value for about 400 seconds after which it showed a slow cooling trend.
titanium airfoil center body after 500 seconds of cloud exposure As the ice thickness grew, the thermocouple became further removed (Figure 10J). Table 7 shows the growth rate to 0.004 mm/s. In addition, from the impinging flow but thermal conduction into the metal surface there was some accretion visible on the aluminum extensions (see may still have played a role as suspected in this case. The other Figures 10 C & G). In this case, the accretion started at the leading thermocouples all reached several degrees below freezing, similar to edge of the aluminum airfoil extension and was loosely adhered. Once Cases (a) and (b). However, the temperature data showed several reaching an appreciable size, the aerodynamic forces pushed the inflection points in TC06 and TC10 which require further analysis and accretion to the top of the airfoil where it anchored for a period of time interpretation to explain. Furthermore, the leading edge ice growth before shedding away. The accretions on the aluminum airfoil (Figure 13J, gray line) had several inflection points resulting in four extensions, although presented on the Ice Shape Analysis Graph different leading-edge growth rate regimes as shown in Table 7. Closer (Figure 10 I), should not be considered for ice accretion code inspection of Figures 12 B and F shows that ice growth initiated development since the impinging cloud and surface conditions are not downstream of the leading edge both above and below the airfoil and well characterized beyond the titanium airfoil center body. Finally, the propagated upstream. It was not until after these upper and lower thermocouple data (Figure 10 J) shows that all the surface growths reached the leading edge and merged that there was significant temperatures decrease to below freezing temperatures although at leading edge growth which began at 240 seconds after spray initiation different rates due to the thermal inertia of the airfoil. Generally, the and continued until the end of the test over 360 seconds later.
thermocouples nearest the leading edge equilibrate faster than those downstream. The leading edge thermocouple, TC03, measures Figure 13 shows the ice accretion for Case (d), which had an MR,e of approximately 0 C shortly after spray initiation staying at this value 0.51 at this condition. Of all the cases studied in this paper, Case (d) for ~30 seconds before cooling to -0.5 C which is warmer than the Page 6 of 11 had the largest size accretion reaching a leading edge thickness of airfoil or due to the presence of supercooled water droplets impinging 33.5 mm (Figure 13 J, gray line). In addition, this case had the largest on the airfoil. The latter possibility is inferred since recent modelling growth rates measured reaching 0.081 mm/s for the second half of the work [26-28] indicates that the water droplets approach the static wet- test (Table 7). Qualitatively, the growth rate and temperature trends bulb temperature which were below freezing for all the cases in the for Case (d) resemble Case (c). This includes that the faster ice growth present experiment. However, it is not clear if the water droplets were initiated aft of the leading edge and then propagated forward. supercooling since the water used during the tests was non-deionized tap water which helps promote freeze-out. Should the liquid portion of the cloud be supercooling then a maximum melt ratio limit may not be
Discussion
reached in the current experiments. Further investigation is required to understand if the cloud was supercooling and to what degree.
There are four primary points to discuss regarding the results of this paper. The first is that small changes in plenum relative humidity can produce large changes in melt ratio at the test section. Starting at a plenum relative humidity of 20% resulted in a melt ratio of 0.14.
Increasing the relative humidity in 5% consecutive increments resulted in melt ratios of 0.21, 0.28, and 0.51, respectively. There is a delicate balance between convective and evaporative cooling that is a function of the several factors such as tunnel speed, temperature, pressure, Leading Edge relative humidity and particle size distribution that ultimately dictates the resulting cloud melt ratio at the tunnel exit.
Ice Growth Rate (mm/s) The second point is that it is not possible in the present experimental setup to vary melt ratio independently of other parameters at the test section by just varying plenum relative humidity. This includes Figure 9. This figure shows the ice growth rates at the leading edge parameters like total water content, total temperature, and wet-bulb versus melt ratio. The blue symbols show the growth rate early in the temperature since they are also a functions of the plenum relative test. The red symbols are from the latter parts of the same test where humidity. For the cases presented, and despite the same amount of the growth rate accelerated at the leading edge for the higher melt cloud spray being injected into the facility, the measured total water ratio cases.
content increased by ~15% for the highest plenum humidity compared with the lowest due to decreased evaporation. However, the total temperature variation was only ~2 C for the cases presented.
Summary/Conclusions
The third discussion point is that the different melt fractions resulted This paper describes an ice-crystal icing experiment conducted at the in different ice shapes particularly when comparing Case (b) to Cases NASA Propulsion System Laboratory during June 2018. This test (c) and (d), the latter two showing qualitatively similar characteristics.
produced ice shape data on an airfoil at four different melt ratios for The lower melt ratio cases appeared smaller, smooth, pointy, and otherwise the same aero-thermal conditions and particle size devoid of feathers. The higher melt ratio cases were generally larger distribution in an upstream plenum. The different melt ratios were than lower melt ratio cases and did have some rougher, feather-like achieved by varying the plenum relative humidity which changed the features downstream of the leading edge. Furthermore, the steady-state amount of evaporative cooling. Despite being at the same plenum growth rates (i.e. those after the initial transients) were still increasing conditions, except for the relative humidity in the plenum, the test at the maximum melt ratios tested (Figure 9). This result is contrary to section total water content, particle size distribution, total and wet-bulb other tests using different test articles reported in the literature which temperatures varied at the test section to different degrees due to the produced ice shapes which stopped growing (i.e. steady-state shapes) different amounts of evaporation from the cloud. The test section despite continuous exposure to the cloud [4, 25].
conditions were measured using a newly developed instrument traverse system which is also described in the paper. The test Finally, in ice crystal icing, there is a general acceptance that a conditions – as well as key ice accretion data like 2D ice shape, growth minimum and maximum melt ratio exists for accretion to occur. The rate, and surface temperature – are documented in the paper for the minimum limit is described as the limit below which too little melt purpose of developing and validating ice-crystal accretion models. The occurs preventing the ice to stick and the ice crystals simply bounce accretions presented support a minimum melt ratio threshold to allow off the surface without accreting, or the impinging ice crystals erode accretion and the existence of a plateau region where there is away any accreted ice. The maximum limit is described as the limit aggressive ice accretion as described by previous research. However, above which there is too much melt and the impinging ice and water a maximum melt ratio limit was not observed during the current tests mixture washes away without accreting. Earlier data (e.g. [4]) suggests and are complicated by a possibility that the liquid portions of the that, in between these limits, aggressive growth is possible. That region water may have supercooled. Furthermore, no steady-state ice shapes is termed a plateau region since variations in melt ratio do not were observed during the present testing.
significantly affect the growth rate. Figure 9 plots the growth rate data as a function of 𝜂 using the data from this experiment. For the ெோ ,
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Acknowledgments
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The authors wish to acknowledge the financial support for this work [16] Strapp, J. W., Lilie, L. E., Ratvasky, T. P., Davison, C. R., et al., by the Advanced Aircraft Icing (AAI) subproject of the NASA "Isokinetic TWC Evaporator Probe: Development of the IKP2 and Advanced Air Transport Technology Project (AATT) under NASA's Performance Testing for the HAIC-HIWC Darwin 2014 and Advanced Air Vehicles Program.
Cayenne Field Campaigns," American Institute of Aeronautics and Astronautics, AIAA-2016-4059 and NASA TM-2016-219151,
Definitions & Abbreviations
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[17] Davison, C. R., Strapp, J. W., Lilie, L. E., Ratvasky, T. P., et al., "Isokinetic TWC Evaporator Probe: Calculations and Systemic BH Background humidity Error Analysis," American Institute of Aeronautics and Astronautics, AIAA-2016-4060, 2016, doi:10.2514/6.2016-4060. CE Collision efficiency [18] Agui, J. H., Struk, P. M., and Bartkus, T. P., "Total Temperature d XX percentile cumulative volume diameter v0.XX Measurements Using a Rearward Facing Probe in Supercool Liquid Droplet and Ice Crystal Clouds," American Institute of HSI High Speed Imaging probe Aeronautics and Astronautics, AIAA-2018-3970, 2018, IKP2 Isokinetic probe, second generation doi:10.2514/6.2018-3970.
[19] King, M. C., Manin, J., Van Zante, J. F., Timko, E. N., et al., LWC Liquid water content "Particle Size Calibration Testing in the NASA Propulsion System Page 8 of 11 Liquid water content, measured using multi- LWC m,0.5 element probe’s 0.5-mm diameter wire Liquid water content, measured using multi- LWC m,2.1 element probe’s 2.1-mm diameter wire MMR Mass mixing ratio Mass mixing ratio, delta, cloud-on minus cloud-off MMR values, = MMR – MMR e,on e,off MVD Median volumetric diameter of cloud MW Multi-wire P Pressure PDI Phase Doppler Interferometer probe PSD Particle size distribution PSL Propulsion Systems Laboratory RFP Rearward facing probe RH Relative humidity (%) T Temperature, air TAT Total Air Temperature (probe or instrument) TC Thermocouple Temperature, delta, cloud-on minus cloud-off values, = T 0,on - T 0,off TPL Total temperature in the plenum T wb Wet-bulb temperature TWC Total water content 𝒕 Leading edge ice thickness U Velocity - bulk, test section e WE Water-catch efficiency MR Melt ratio (dimensionless) Subscripts 0 Total or plenum conditions e Exit or test section conditions i Inlet condition (at spray bars) m Measured value off Cloud-off on Cloud-on s Static condition T Target value Page 9 of 11 Ice Shape Analysis Graph Titanium A I E Center Ice on Body Extension TC10 t = 60 s TC06 Aluminum TC03 Extensions Ice on Extension B F Flow t = 180 s Leading Edge Flow C G J Surface Temperature and Ice Thickness Graph t = 300 s Ice on Extension 10.5 cm D H t = 493 s Span Camera Views Side Profile Camera Views Figure 10. Ice Accretion Images (Panels A-H) and Profile Analysis (Panel I) for Case (a) - Escort 170 (RH 0,i =20%, MR,e =0.14). Panel J shows the time histories of surface temperature (colored lines) and leading edge ice thickness (gray hatched line). The green vertical lines in Panel J correspond to the times when the images on the left were taken.
Ice Shape Analysis Graph A E I t = 60 s TC10 TC06 TC03 B F t = 180 s Surface Temperature and Ice Thickness Graph C G J t = 360 s Ice Particle Debris Field D H t = 600 s Figure 11. Ice Accretion Images (Panels A-H) and Profile Analysis (Panel I) for Case (b) - Escort 593 (RH = 25%, =0.21). Panel J shows the 0,i MR,e time histories of surface temperature (colored lines) and leading edge ice thickness (gray hatched line). The green vertical lines in Panel J correspond to the times when the images on the left were taken.
Page 10 of 11 Ice Shape Analysis Graph A E I t = 60 s TC10 TC06 TC03 B F t = 180 s Surface Temperature and Ice Thickness Graph C G J t = 300 s H D t = 600 s Figure 12. Ice Accretion Images (Panels A-H) and Profile Analysis (Panel I) for Case (c) - Escort 171 (RH = 30%, = 0.28). Panel J shows the 0,i MR,e time histories of surface temperature (colored lines) and leading edge ice thickness (gray hatched line). The green vertical lines in Panel J correspond to the times when the images on the left were taken.
Ice Shape Analysis Graph A E I t = 60 s TC10 TC06 TC03 B F t = 180 s Surface Temperature and Ice Thickness Graph G J C t = 300 s D H t = 600 s Figure 13. Ice Accretion Images (Panels A-H) and Profile Analysis (Panel I) for Case (d) - Escort 169 (RH = 35%, =0.51). Panel J shows the 0,i MR,e time histories of surface temperature (colored lines) and leading edge ice thickness (gray hatched line). The green vertical lines in Panel J correspond to the times when the images on the left were taken.
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